The CMOS SRAM, in which are combined a high resistance load type or complete CMOS (Complementary .Metal-Oxide-Semiconductor) type memory cell and a peripheral circuit composed of a complementary MISFET (Metal-Insulator-Semiconductor Field-Effect-Transistor) (CMOSFET), has been used for a cache memory of a computer or workstation of the prior art.
The memory cell of the CMOS SRAM is composed of a flip-flop circuit for storing information of 1 bit, and two transfer MISFETs. The flip-flop circuit of the high resistance load type is composed of a pair of driver MISFETs and a pair of resistance elements, whereas the flip-flop circuit of the complete CMOS type is composed of a pair of driver MISFETs and a pair of load MISFETs.
In recent years, the SRAM of this kind has been required to miniaturize the memory cell size to increase the capacity and speed and to lower the operating voltage to reduce the power consumption of the system. However, to meet the requirement, a problem that the resistance to soft error due to alpha rays (d-ray) must be solved.
The soft error due to alpha rays is a phenomenon that alpha rays (He nuclei) contained in cosmic rays or emitted from radioactive atoms contained in the resin materials of LSI packages, come into the memory cell to break the information retained in the information storage section.
An alpha particle has an energy of 5 eV and produces an electron-hole pair when it is incident upon the silicon (Si) substrate. When an alpha ray comes into a storage node at a "High" potential level, of the memory cell, the electron produced by the alpha-ray, flows to the storage nodes so that the hole flows to the substrate. As a result, the charge and potential of the storage node instantly decrease to invert the information of the memory cell with a certain probability.
In the case of an SRAM, the increase in the storage node capacitance of the memory cell is effective in improving the aforementioned resistance to soft error due to alpha rays.
U.S. Pat. No. 5,483,083 discloses a TFT (Thin Film Transistor) complete CMOS SRAM in which the load MISFETs are made of two-layered polycrystalline silicon film formed over the driver MISFET. In the SRAM, as disclosed, the gate electrode of one of the load MISFETs is partially extended to above the source or drain region of the other of the load MISFETs, and a capacitor element is formed of the gate electrode, the source or drain region and a insulating film interposed between the former two so that the storage node capacitance may be increased.